A collection of light sources such as multiple light-emitting diodes (hereinafter called “LEDs”) and lasers connected in parallel has been suggested as a light source for a projection type image display device. Connecting LEDs in parallel makes it possible to drive a large number of LEDs at a low voltage. Further, making multiple LEDs light up allows acquisition of high-luminance light and reduction in the power consumption of the entire device compared to a lamp light source of a conventional type.
As an applicable device to control light-up of multiple LEDs, a device sets a driving current to be supplied from a constant current circuit to the LEDs connected in parallel by a controller such as a microcomputer. Luminance determined while the LED light up changes in response to the driving current supplied to the LEDs. Thus, a user can obtain light of a desired luminance by controlling the driving current with the microcomputer. Japanese Patent Application Laid-Open Nos. 2007-095391 and 2007-096113 disclose techniques of setting a driving current with a controller such as a microcomputer to adjust the luminance of LEDs.
In a structure with multiple parallel-connected LEDs, a short-circuit failure occurring even in one LED hinders flow of a current into a rest of LED of a higher resistance than the LED subjected to the short-circuit failure. This disables light-up of an LED not subjected to a short-circuit failure.
If the device continues to be used while one of multiple LEDs goes out due to its short-circuit failure, for example, a current is supplied intensively to the LED subjected to the short-circuit failure from the constant current circuit. This becomes not only a cause for the aforementioned disabling of light-up of many LEDs but it is considered to also become a cause for a failure to occur in a rest of part of the device due to temperature increase resulting from heat generated by the LED subjected to the short-circuit failure.